Bioconversion of cellulose into glucose is an environmental approach to withdraw useful products from cotton based waste textile. Pretreatment of the feedstock is a primary and critical step to increase the cellulose accessibility to cellulase before enzymatic hydrolysis. Many pretreatment processes have been developed to reduce the recalcitrance of textile waste to enzymatic hydrolysis and furthermore decrease the consumption of cellulase. The basic approaches can be classified into four categories, i.e., chemical, biological, physical, and physico-chemical processes. Different pretreatment processes may result in different types and levels of structural modification and improve enzymatic hydrolysis after different mechanisms.
Acid pretreatment is currently the most widely studied and applied approach for the pretreatment of cellulose fibre and the other related feedstock. The key mechanisms of the acid pretreatment are the decomposition of the microstructure of cellulose fibres. The amorphous region of cellulose can be hydrolyzed while the crystalline regions with more reducing ends and non-reducing ends can be exposed to the enzymes, which facilitates the enzyme to perform degradation process. Acid pretreatment process, however, requires special reactors/container to prevent corrosion at elevated concentrations; and the acid reagents used are difficult to recycle. Both those factors can considerably increase the operation costs and environmental impacts of the overall biorefinery process. Meanwhile, high severity acid pretreatment (i.e., high acid doses, high temperature, and extended reaction time) can promote the conversion of carbohydrates to form into sugar dehydration by-products (i.e., furan-type inhibitors; 5-hydroxy methyl-furfural), which are harmful to the downstream fermentation processes at high concentration.
Another pretreatment method using ionic liquid (ILs) has drawn a significant amount of attention recently. There are mainly two factors lead to the various properties of ILs: the cation structure (the symmetrical array, the influence of alkylphosphonate and hydrophobic groups) and the anion delocalization degree. Some ILs have shown outstanding characteristics for industrial application, i.e. high chemical and thermal stabilities, liquid form in wide range of temperature, low vapour pressure, and low viscosity operation which reduces the cost of mixing. ILs have been tested in pretreating certain cotton or textile wastes, although its feasibility to large scale application is still unclear and needs further investigation. Hong, Guo used a 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) IL to treat un-dyed 100% cotton t-shorts. After 90 minutes of pretreatment under 110° C. a high sugar yield (94%) was achieved by using reasonable amount of cellulase. De Silva et al. used the same IL to treat a 50:50 blend PET/cotton yard at a longer cooking period (6 hours) and higher temperature (120° C.). The cotton was effectively dissolved and regenerated in the anti-solvent and formed into fibre films after regeneration. The PET was completely recovered after the process. The key limiting factors hindering the applicability of ILs are high production cost and environmental toxicity. The current price for ([AMIM]Cl) IL is approximately US$22-26 per gram (chemical grade, Sigma-Aldrich). Furthermore, ILs have shown substantial negative influence on enzyme hydrolysis while the IL/cellulose mixture is difficult to handle in the existing reactor systems due to its high viscosity.
4-methylmorpholine 4-oxide (NMMO) is another solvent that can dissolve cellulose and provide reasonable yield. In NMMO process, heating at 80° C. under low water content leads to dissolution of higher molecular weight cotton fibres. However, NMMO concentrations above 5 and 25 g/L showed inhibition effects on enzymatic hydrolysis and fermentation. The treated materials must be washed before enzymatic hydrolysis and techno-economic studies showed that efficient recycling of NMMO is required in order to have an economically feasible process for pretreatment of cotton materials with NMMO. Certain side reactions present in the Lyocell process might affect the pretreatment system as well and lead to decomposition of NMMO, furthermore increasing the consumption of stabilizer. On the other hand, efficient removal of NMMO from the treated material by washing requires high amounts of water. This significant amount of water must be evaporated in an energy consuming process before reusing NMMO, which is an energy intensive process.
Supercritical fluid is a phase of reagents of which both gas and liquid phases coexist under a specific pressure and temperature. It shows liquid like density and gas like diffusing/penetrating ability to solid materials. Among all the supercritical fluids, supercritical carbon dioxide, which has a critical temperature at 31° C., has shown to be suitable for pretreatment of cellulosic feedstock. Supercritical carbon dioxide (SC—CO2) has been widely used as an extraction solvent. In aqueous solution, CO2 forms carbonic acid and can improve the hydrolysis of polymers. CO2 molecule is similar in size as the molecules of water or ammonia, and therefore, they can penetrate through the same pathway to the small pores of the cellulose. CO2 has even been used to modify steric structures of some cellulases to improve their stability, solvent tolerance and reactivity. Saka and Ueno investigated the direct conversion of various types of celluloses (including cotton linter) in supercritical water (500° C., 35 MPa) into glucose and found that cellulose can be hydrolyzed to a similar level as acid or enzymatic hydrolysis, without the difficulties associated with hydrolysing techniques. Muratov and Kim studied the performance of enzymatic hydrolysis of cotton fibres in supercritical CO2 (120 atm, 50° C., 48 h) and found that the productivity of the glucose increased by 20% as compared to atmospheric conditions. As commercial scale of supercritical CO2 treatment has been seen in textile industries (such as for dyeing), similar technology and equipment could be worthwhile to be explored further for the treatment of textile wastes in the future.
Alkaline pretreatment of textile waste can also be performed. During alkaline pretreatment, the first reactions that occur are solvation and saponification, of intermolecular ester bonds cross-linking which swells the textile to expose more accessible areas for the enzymes to hydrolyse the cellulose. However, a noticeable disadvantage of alkaline pretreatment is the formation of irrecoverable salts from alkaline or the combination of salts into the biomass which hinders the enzymatic hydrolysis. And alkaline pretreatment also works as another mechanism at ambient temperature at longer pretreatment time. At high alkaline concentration dissolution step, the effect of alkaline hydrolysis surpasses the alkaline dissolution, the ‘peeling’ of end-groups also hydrolyse the dissolved cellulose through degradation and decomposition cause the loss of polysaccharides. This increases the chance of loss of carbon, converting to carbon dioxide.
With all of the known pretreatment methods, the amount of enzyme loading required for the hydrolysis step is typically 20-30 FPU/g glucan. Accordingly, there remains a need for an improved textile waste treatment method that is more efficient, cheaper and which requires a lower enzyme loading.